How this calculator works
A photon carries energy h times its frequency. In vacuum, wavelength and frequency multiply to c, so energy is also hc divided by wavelength. Use vacuum wavelength for the energy conversion. In a material, photon frequency remains the useful quantity because phase wavelength also depends on refractive index. Energy here is per photon, rather than power or a beam energy total. All defining constants are the exact SI values listed in CODATA 2018.
Formula
E = hf = hc/lambda; 1 eV = 1.602176634e-19 J. In eV and nm: E = 1239.841984332.../lambda.
Worked example
A 500 nm photon carries about 2.479683969 eV or 3.972891714e-19 J. Doubling its wavelength halves its energy.
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Frequently asked questions
Is this energy per mole?
No. Multiply the per-photon value by Avogadro constant for energy per mole of photons.
Can I enter wavelength measured in water?
Convert it to vacuum wavelength using the phase index, or use the measured frequency directly.
Why show a coefficient and exponent?
A tiny joule value can round to zero in an ordinary decimal readout. Scientific notation preserves its scale.